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In the simplest of terms, a combined
cycle power plant burns natural gas or liquid fuel in a
combustion turbine to
drive an electrical generator. Waste heat from this turbine's
exhaust
is passed
through a large duct or heat recovery generator (HRSG) filled with
water tubes.
Heat from
the exhaust creates steam from the water passing through the tubes
and collects
in steam
drums atop the duct assembly. The steam is produced in multiple
pressure levels
with the
highest-pressure steam being delivered to a steam turbine that
drives an additional
generator.
Power produced by the generators is
applied to transformers where the voltage is stepped
up to the level of
the utility grid. From the transformer switchyard the power is
exported to
consumers
via the utility transmission lines. Auxiliary power is also stepped
down to
lower
voltage levels for use in energizing the plant's parasitic loads,
necessary for
operation.
In addition to this process the
combined cycle plant is comprised of many auxiliary systems
that support the
operation of each major component and the balance of plant systems.
The
most complex and
dynamic of these systems comprise the steam cycle. The steam cycle
begins and ends at the surface condenser or the heat exchanger
whose
function it is to condense the steam exhausting from the steam
turbines low-pressure section.
As the steam enters the condenser from
the turbine's LP section, it is condensed by cooling
water being
circulated through the internal tubes from large pumps locate at the
cooling
tower.
Heat from the condensed steam is transferred to the water returning
to the cooling
tower
risers where it is dissipated via the tower fans before falling to
the tower basin.
Condensed steam or condensate is
collected in the condenser hotwell from where it is
pumped to the water
tubes located in the coolest end of the HRSG. Here it is preheated
in
economizer modules.
The water then flows forward through multiple banks of tubes
towards the heat
source - that being the turbine exhaust at the opposite end. At
certain
points in the duct
structure, the tubes form evaporator modules that are connected to
the
steam drums. As the
water enters the steam drum from the evaporator, it flashed into
steam
and is
driven through steam tubes upstream from the evaporator tubes. As
the steam takes
a final
pass through the HRSG duct it becomes superheated and enters the
steam
distribution
piping on its path to the steam turbine.
Entering the steam turbine, the
high-pressure steam releases its energy in the HP section of
the turbine driving
the generator. This steam then exits the turbine as cold reheat
steam
and takes another
pass through the HRSG. Heated once again, it reenters the steam
turbine
as hot reheat steam
and is passed through the turbine to the low-pressure end where it
combines
with low-pressure steam from the HRSG's low-pressure drum. The LP
steam
then
enters the crossover tube of the turbine to the low-pressure section
where it passes to
the
surface condenser completing the steam cycle.
Combined cycle plants typically have
multiple "trains" or the grouping of a combustion
turbine
generator and an HRSG with the associated auxiliary support systems.
Multiple
trains
can supply steam to a single steam turbine in various configurations
identified as
2X1 or
3X1, etc. Each combination constitutes a power block of which the
largest plants
may have
several. Common equipment that support the entire plant such as
plant air and
water
systems are identified as "balance of plant systems".
The design of the combined cycle plant
is generally based on continuous base load
operation
of all power blocks with variations in power output being made in
the operation
of the
individual trains. Each train can be cycled in and out of service or
operated at full
load
depending on the utilities demand for power. Cycling; however, is
costly in both time
and the
wear on the equipment
At base load the combustion turbine
exhaust provides adequate heat and motive force to produce steam at
a temperature and pressure to operate the steam turbine. When the
utilities' power demand increases; however, the operator utilizes
the gas fired duct burner located in the HRSG to increase exhaust
temperature, thus maximizing steam production to the steam turbine.
Power output at this point is determined by load demand on the steam
turbine, which in turn regulates the amount of steam supplied to the
turbine to maintain load.
In support of this operation, many
auxiliary systems are used to cool, lubricate, chemically treat and
condition the fluids and gasses necessary to operate the plant.
Balance of plant systems found in the referenced plant designs are
for the most part, common to all and are similar in design and
function.
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